IETF Security Tutorial
Radia Perlman
Intel Labs
March 2010
([email protected])
1
Why an IETF Security Tutorial?
• Security is important in all protocols; not
just protocols in the security area
• IETF specs mandated to have a “security
considerations” section
• There is no magic security pixie dust where
you can ignore security and then plug in a
security considerations section
2
Know the tools…
• If your protocol runs over TCP, you
(mostly) don’t need to worry about message
retransmission and congestion control
• If your protocol runs over IP, you (mostly)
don’t need to worry about routing
• Isn’t there something similar for security?
3
Sometimes
• If your protocol runs over SSL/TLS, or IPSEC,
and it’s reasonable to expect both ends will have
appropriate credentials, you might not have to
worry about security
• You’re unlikely to be this lucky
– Appropriate credentials
– A lot of interesting protocols run at layer 3 or below
(and IPsec depends on layer 3 and below)
4
What’s hard about credentials?
• Security infrastructure rollout is late. You
will probably need something lightweight as
an optional alternative.
• Using the credentials from SSL and IPsec is
not always easy or appropriate.
5
Purpose of this tutorial
• A quick intro into a somewhat scary field
• A description of what you need to know vs.
what you can trust others to do
• An overview of the security WGs
• Cross-fertilization: there’s no cookbook for
any area, and different areas need to learn
from each other
6
A plea for cross-fertilization
• The best way to get advice, say from a
security expert, is to make it easy for them
to come up to speed on your protocol
• Summarizing, explaining things at
conceptual levels, is not a waste of time,
even for those that (think they) understand
the protocol
7
Agenda
• • • • Introduction to Security
Introduction to Cryptography
Authenticating People
Security mechanisms to reference rather than
invent
– Public Key / Secret Key infrastructures
– Formats
• Security Considerations Considerations
• Security Working Groups
8
The Problem
• Internet evolved in a world w/out predators. DOS
was viewed as illogical and undamaging.
• The world today is hostile. Only takes a tiny
percentage to do a lot of damage.
• Must connect mutually distrustful organizations
and people with no central management.
• And society is getting to depend on it for
reliability, not just “traditional” security concerns.
9
Security means different things to
different people
• • • • Limit data disclosure to intended set
Monitor communications to catch terrorists
Keep data from being corrupted
Make sure nobody can access my stuff without
paying for it
• Destroy computers with pirated content
• Track down bad guys
• Communicate anonymously
10
Insecurity
The Internet isn’t insecure. It may be unsecure.
Insecurity is mental state. The users of
the Internet may be insecure, and perhaps
rightfully so……Simson Garfinkel
11
Intruders: What Can They Do?
• Eavesdrop--(compromise routers, links, routing
algorithms, or DNS)
• Send arbitrary messages (including IP hdr)
• Replay recorded messages
• Modify messages in transit
• Write malicious code and trick people into running
it
• Exploit bugs in software to ‘take over’ machines
and use them as a base for future attacks
12
Some basic terms
• Authentication: “Who are you?”
• Authorization: “Should you be doing that?”
• DOS: denial of service
• Integrity protection: a checksum on the data
that requires knowledge of a secret to
generate (and maybe to verify)
13
Some Examples to Motivate the
Problems
• Sharing files between users
– File store must authenticate users
– File store must know who is authorized to read
and/or update the files
– Information must be protected from disclosure
and modification on the wire
– Users must know it’s the genuine file store (so
as not to give away secrets or read bad data)
– Users may want to know who posted the data in
the file store
14
Examples cont’d
• Electronic Mail
– Send private messages
– Know who sent a message (and that it hasn’t
been modified)
– Non-repudiation - ability to forward in a way
that the new recipient can know the original
sender
– Anonymity
– Virus Scanning
– Anti-spam
15
Examples cont’d
• Electronic Commerce
– Pay for things without giving away my credit
card number
• to an eavesdropper
• or phony merchant
– Buy anonymously
– Merchant wants to be able to prove I placed the
order
16
Examples, cont’d
• Routing protocol
– Handshake with neighbor
• Is the message from a valid router? (replay?)
• How do we recognize a valid router?
(autoconfiguration incompatible with security)
– Routing messages
• Even valid routers might lie (become subverted)
– Forwarding (which can also be DDOS’d)
17
Sometimes goals conflict
• privacy vs. company (or govt) wants to be
able to see what you’re doing
• losing data vs. disclosure (copies of keys)
• denial of service vs. preventing intrusion
• privacy vs. virus scanning
18
Agenda
• • • • Introduction to Security
Introduction to Cryptography
Authenticating People
Security mechanisms to reference rather than
invent
– Public Key / Secret Key infrastructures
– Formats
• Security Considerations Considerations
• Security Working Groups
19
Cryptography
• It’s not as scary as people make it out to be
• You don’t need to know much about it to
understand what it can and can’t do for you
20
Features
• Main features
– Encryption
– Integrity protection
– Authentication
• More things
– Denial of service defense
– Nonrepudiation
– Perfect forward secrecy
21
Cryptography
• Three kinds of cryptographic algorithms
you need to understand
– secret key
– public key
– cryptographic hashes
• Used for
– authentication, integrity protection, encryption
22
Secret Key Crypto
• Two operations (“encrypt”, “decrypt”)
which are inverses of each other. Like
multiplication/division
• One parameter (“the key”)
• Even the person who designed the algorithm
can’t break it without the key (unless they
diabolically designed it with a trap door)
• Ideally, a different key for each pair of users
23
Secret key crypto, Alice and Bob
share secret S
• encrypt=f(S, plaintext)=ciphertext
• decrypt=f(S, ciphertext)=plaintext
• authentication: send f(S, challenge)
• integrity check: f(S, msg)=X
• verify integrity check: f(S, X, msg)
24
A Cute Observation
• Security depends on limited computation
resources of the bad guys
• (Can brute-force search the keys)
– assuming the computer can recognize plausible
plaintext
• A good crypto algo is linear for “good guys” and
exponential for “bad guys”
• Faster computers work to the benefit of the good
guys!
25
Public Key Crypto
• Two keys per user, keys are inverses of each
other (as if nobody ever invented division)
– public key “e” you tell to the world
– private key “d” you keep private
• Yes it’s magic. Why can’t you derive “d”
from “e”?
• and if it’s hard, where did (e,d) come from?
26
Digital Signatures
• One of the best features of public key
• An integrity check
– calculated as f(priv key, data)
– verified as f(public key, data, signature)
• Verifiers don’t need to know secret
• vs. secret key, where integrity check is
generated and verified with same key, so
verifiers can forge data
27
Cryptographic Hashes
• Invented because public key is slow
• Slow to sign a huge msg using a private key
• Cryptographic hash
– fixed size (e.g., 160 bits)
– But no collisions! (at least we’ll never find one)
• So sign the hash, not the actual msg
• If you sign a msg, you’re signing all msgs
with that hash!
28
Example Secret Key Algorithms
• DES (old standard, 56-bit key, slow,
insecure)
• 3DES: fix key size but 3 times as slow
• RC4: variable length key, “stream
cipher” (generate stream from key, XOR
with data), really fast, stream sometimes
awkward
• AES: replacement for DES
29
Example Public Key Algorithms
• RSA: nice feature: public key operations
can be made very fast, but private key
operations will be slow. Patent expired.
• ECC (elliptic curve crypto): smaller keys,
so faster than RSA.
30
Crypto-agile
• Notice all the crypto algorithms
• The cryptographers can tell you at any time
that the one you picked isn’t good
• So you have to design your protocols to be
able to switch crypto algorithms
• Which means for interoperability your
protocol has to do negotiation
31
Encrypting with public key
Instead of:
Message
Encrypted with Alice’s Public Key
Use:
Randomly
Chosen K
Encrypted with
Alice’s Public Key
Message
+
Encrypted with
Secret Key K
32
Digital Signatures
Instead of:
Message
Signed with Bob’s Private Key
Use:
Message
+
Message
Digest
(Message)
Signed with Bob’s Private Key
33
Signed and Encrypted Message
Randomly
Chosen K
Encrypted with
Alice’s Public Key
Message+
+
Digest (Message)
Signed with
Bob’s Private Key
Encrypted with
Secret Key K
34
Don’t try this at home
• No reason (except for the Cryptography
Guild) to invent new cryptographic
algorithms
• Even if you could invent a better (faster,
more secure) one, nobody would believe it
• Use a well-known, well-reviewed standard
35
Challenge / Response
Authentication
Bob (knows K)
Alice (knows K)
I’m Alice
Pick Random R
Encrypt R using K
(getting C)
If you’re Alice, decrypt C
R
36
Non-Cryptographic Network
Authentication (olden times)
• Password based
– Transmit a shared secret to prove you know it
• Address based
– If your address on a network is fixed and the
network makes address impersonation difficult,
recipient can authenticate you based on source
address
– UNIX .rhosts and /etc/hosts.equiv files
37
Agenda
• • • • Introduction to Security
Introduction to Cryptography
Authenticating People
Security mechanisms to reference rather than
invent
– Public Key / Secret Key infrastructures
– Formats
• Security Considerations Considerations
• Security Working Groups
38
Authenticating people
• What you know (passwords)
• What you have (smart cards, SecurID cards,
challenge/response calculators)
• What you are (biometrics)
39
Passwords are hard to get right!
• People “can’t” remember passwords with
enough cryptographic strength to provide
meaningful security as keys
• People reuse passwords, so it is dangerous
to have servers storing passwords for their
users
• Turn user authentication into real keys as
close to the user as possible
40
People
• “Humans are incapable of securely storing high-quality
cryptographic keys, and they have unacceptable speed
and accuracy when performing cryptographic
operations. They are also large, expensive to maintain,
difficult to manage, and they pollute the environment.
It is astonishing that these devices continue to be
manufactured and deployed, but they are sufficiently
pervasive that we must design our protocols around
their limitations.”
– Network Security: Private Communication in a
Public World
41
Passwords ‘in the clear’ considered
harmful
• Assume eavesdropping on the Internet is
universal.
• Surest way to get your protocol bounced by
IESG.
42
On-Line Password Guessing
• If guessing must be on-line, password need only
be mildly unguessable
• Can audit attempts and take countermeasures
– ATM: eat your card
– military: shoot you
– networking: lock account (subject to DOS) or be
slow per attempt
43
Off-Line Password Guessing
• If a guess can be verified with a local
calculation, passwords must survive a very
large number of (unauditable) guesses
44
Passwords as Secret Keys
• A password can be converted to a secret key
and used in a cryptographic exchange
• An eavesdropper can often learn sufficient
information to do an off-line attack
• Most people will not pick passwords good
enough to withstand such an attack
45
Off-line attack possible
Alice
(knows pwd)
Workstation
Server
(knows h(pwd))
“Alice”, pwd
Compute h(pwd)
I’m Alice
R (a challenge)
{R}h(pwd)
46
Other ways of authenticating
people
• OTP
• Tokens (e.g., challenge/response, timebased)
• SASL and EAP are frameworks for
negotiating what kind of authentication to
do
47
Trusted Third Parties
48
How do two parties get introduced?
49
Key Distribution - Secret Keys
• Could configure n2 keys
• Instead use Key Distribution Center (KDC)
– Everyone has one key
– The KDC knows them all
– The KDC assigns a key to any pair who need to
talk
• This is basically Kerberos
50
KDC
Alice/Ka
Alice/Ka
Bob/Kb
Carol/Kc
Ted/Kt
Fred/Kf
Ted/Kt
Bob/Kb
Fred/Kf
Carol/Kc
51
Key Distribution - Secret Keys
KDC
Alice
A wants to talk to B
Bob
Randomly choose Kab
{“B”, Kab}Ka
{“A”, Kab}Kb
{Message}Kab
52
Key Distribution - Public Keys
• Certification Authority (CA) signs
“Certificates”
• Certificate = a signed message saying “I, the
CA, vouch that 489024729 is Radia’s public
key”
• If everyone has a certificate, a private key,
and the CA’s public key, they can
authenticate
53
Key Distribution - Public Keys
Bob
Alice
[“Alice”, key=342872]CA
[“Bob”, key=8294781]CA
Auth, encryption, etc.
54
KDC vs CA Tradeoffs
• KDC solution less secure
– Highly sensitive database (all user secrets)
– Must be on-line and accessible via the net
• complex system, probably exploitable bugs,
attractive target
– Must be replicated for performance, availability
• each replica must be physically secured
55
KDC vs CA
• KDC more expensive
– big, complex, performance-sensitive, replicated
– CA glorified calculator
• can be off-line (easy to physically secure)
• OK if down for a few hours
• not performance-sensitive
• Performance
– public key slower, but avoid talking to 3rd party
during connection setup
56
KDC vs CA Tradeoffs
• CA’s work better interrealm, because you
don’t need connectivity to remote CA’s
• Revocation levels the playing field
somewhat
57
Revocation
• What if someone steals your credit card?
– depend on expiration date?
– publish book of bad credit cards (like CRL
mechanism …cert revocation list)
– have on-line trusted server (like OCSP …
online certificate status protocol)
58
Another philosophy: “Identity
Providers”
• For web-based protocols
• Usually based on SAML standard, which
uses XML syntax
• User authenticates to an identity provider
• To authenticate to an affiliated web site, use
the magic of URL rewriting, http
redirection, cookies, etc., to obtain a cookie
signed by the IDP saying “this is Radia”
59
Functional difference from Kerberos
• Although SAML could encode same information
as a (PKI) certificate, the usual use is as a “bearer
token”
• With Kerberos, the “ticket” doesn’t prove who you
are, you have to prove knowledge of the key
inside the ticket
• Likewise, with PKI, you still have to prove
knowledge of the private key associated with the
public key in the certificate
60
Other interesting issues (with all the
schemes)
• You won’t have one KDC/CA/IDP for the
whole world
• So how do you find a proper chain?
61
Strategies for CA Hierarchies
• Monopoly
• Oligarchy
• Anarchy
• Bottom-up
62
Monopoly
• Choose one universally trusted organization
• Embed their public key in everything
• Give them universal monopoly to issue
certificates
• Make everyone get certificates from them
• Simple to understand and implement
63
What’s wrong with this model?
• Monopoly pricing
• Getting certificate from remote organization
will be insecure or expensive (or both)
• That key can never be changed
• Security of the world depends on honesty
and competence of that one organization,
forever
64
One CA Plus RAs
• RA (registration authority), is someone
trusted by the CA, but unknown to the rest
of the world (verifiers).
• You can request a certificate from the RA
• It asks the CA to issue you a certificate
• The CA will issue a certificate if an RA it
trusts requests it
• Advantage: RA can be conveniently located
65
What’s wrong with one CA plus
RAs?
• Still monopoly pricing
• Still can’t ever change CA key
• Still world’s security depends on that one
CA key never being compromised (or
dishonest employee at that organization
granting bogus certificates)
66
Oligarchy of CAs
• Come configured with 80 or so trusted CA
public keys (in form of “self-signed”
certificates!)
• Usually, can add or delete from that set
• Eliminates monopoly pricing
67
Default Trusted Roots in IE
68
What’s wrong with oligarchy?
• Less secure!
– security depends on ALL configured keys
– naïve users can be tricked into using platform
with bogus keys, or adding bogus ones (easier
to do this than install malicious software)
– impractical for anyone to check trust anchors
• Although not monopoly, still favor certain
organizations. Why should these be trusted?
69
CA Chains
• Allow configured CAs to issue certs for
other public keys to be trusted CAs
• Similar to CAs plus RAs, but
– Less efficient than RAs for verifier (multiple
certs to verify)
– Less delay than RA for getting usable cert
70
Anarchy
• Anyone signs certificate for anyone else
• Like configured+delegated, but user consciously
configures starting keys
• Problems
– won’t scale (computationally too difficult to
find path)
– no practical way to tell if path should be
trusted
– too many decisions for user
71
Top Down with Name
Subordination
• Assumes hierarchical names
• Each CA only trusted for the part of the
namespace rooted at its name
• Can apply to delegated CAs or RAs
• Easier to find appropriate chain
• More secure in practice (this is a sensible
policy that users don’t have to think about)
72
Bottom-Up Model
• Each arc in name tree has parent certificate (up)
and child certificate (down)
• Name space has CA for each node
• “Name Subordination” means CA trusted only for
a portion of the namespace
• Cross Links to connect Intranets, or to increase
security
• Start with your public key, navigate up, cross, and
down
73
Intranet
abc.com
nj.abc.com
[email protected]
[email protected]
ma.abc.com
[email protected]
74
Extranets: Crosslinks
abc.com
xyz.com
75
Extranets: Adding Roots
root
abc.com
xyz.com
76
Advantages of Bottom-Up
• For intranet, no need for outside
organization
• Security within your organization is
controlled by your organization
• No single compromised key requires
massive reconfiguration
• Easy configuration: public key you start
with is your own
77
What formats can you leverage?
• Depends on your “layer in the stack”
• IPsec protects packets
– Could be end to end or between firewalls
– Today, most uses are transparent to applications
• TLS & SSH protect sessions
• OpenPGP, S/MIME and CMS, XML-DSIG
and XML-encryption, protect messages
(needed for store and forward)
78
IPsec vs. TLS
• IPsec idea: don’t change applications or API
to applications, just OS
• TLS idea: don’t change OS, only change
application (if they run over TCP)
• but… unless OS can set security context of
application, server applications need to
know identity of their clients
79
IPsec vs. TLS
• IPsec technically superior
– Rogue packet problem
• TCP doesn’t participate in crypto, so attacker can
inject bogus packet, no way for TCP to recover
– easier to do outboard hardware processing
(since each packet independently encrypted)
• TLS easier to deploy
• And unless API changes, IPsec can’t pass
up authenticated identity
80
Creating a session key
81
An Intuition for Diffie-Hellman
• Allows two individuals to agree on a secret
key, even though they can only
communicate in public
• Alice chooses a private number and from
that calculates a public number
• Bob does the same
• Each can use the other’s public number and
their own private number to compute the
same secret
• An eavesdropper can’t reproduce it
82
Why is D-H Secure?
• We assume the following is hard:
• Given g, p, and gX mod p, what is X?
• With the best known mathematical techniques, this
is somewhat harder than factoring a composite of
the same magnitude as p
• Subtlety: they haven’t proven that the algorithms
are as hard to break as the underlying problem
83
Diffie-Hellman
Alice
agree on g,p
choose random A
Bob
choose random B
gA mod p
gB mod p
compute (gB mod p) A
compute (gA mod p)B
agree on gAB mod p
84
Man in the Middle
Alice
Bob
Trudy
gA mod p
gT mod p
gT mod p
gB mod p
agree on gAT mod p
agree on gTB mod p
{data}gAT mod p
{data}gTB mod p
{data}gAT mod p
{data}gTB mod p
85
Signed Diffie-Hellman
(Avoiding Man in the Middle)
Alice
Bob
choose random A
choose random B
[gA mod p] signed with Alice’s Private Key
[gB mod p] signed with Bob’s Private Key
verify Bob’s signature
verify Alice’s signature
agree on gAB mod p
86
If you have keys, why do D-H?
• “Perfect Forward Secrecy” (PFS)
• Prevents me from decrypting a conversation
even if I break into both parties after it ends
(or if private key is escrowed)
87
Example non-PFS (like SSL)
Bob
Alice
{K}Bob
protect conversation using K
88
PFS without Diffie-Hellman
Bob
Alice
[Use public key P]Bob
invent new RSA pair
for this conversation
{K}P
protect conversation using K
89
Replay Issues
• Usually use a sequence number
• Without unique session key per session, if
start with same initial sequence number, can
do replays
• What if sequence number too small?
90
Extended sequence number
• For instance, TCP’s sequence number is too
small
• Instead of changing the protocol to have a
larger sequence number, do the integrity
check on a larger sequence number,
“pretending” it’s in the packet
91
Agenda
• • • • Introduction to Security
Introduction to Cryptography
Authenticating People
Security mechanisms to reference rather than
invent
– Public Key / Secret Key infrastructures
– Formats
• Security Considerations Considerations
• Security Working Groups
92
Every RFC needs a “security
considerations” section
• What do you have to think about?
• Not enough to say “just use IPsec”
• Sometimes (as with VRRP) protecting one
protocol in a vacuum is wasted effort
– putting expensive locks on one window, while
the front door is wide open
• We don’t need to protect a protocol. We
need to protect the user
93
Things to put in a security
considerations section
• What are the threats? Which are in scope?
Which aren’t? (and why)
• What threats are defended against? Which
are the protocol susceptible
• Implementation or deployment issues that
might impact security
• See RFC 3552 “Guidelines for Writing RFC
Text on Security Considerations”
94
Examples
• Putting integrity checks on routing msgs
– Defends against outsiders injecting routing
msgs. That’s good, but
– Doesn’t prevent outsiders from answering
ARPs, or corrupting DNS info
– Doesn’t protect against “Byzantine
failures” (where a trusted thing goes bad)
95
Examples
• SNMP
• Should be straightforward end-to-end
security
• But it has to work when the network is flaky
– DNS not available
– LDAP database for retrieving certificates might
be down, as might revocation infrastructure
96
Examples
• Non-crypto things
– Use up resources
• DHCP, could request all possible addresses
• Use all bandwidth on a link
– Interface to human – internationalized names
– Active Content
• Too many examples of hidden places for active
content!
97
Examples
• Email (much more detail in RFC 2552), but
some cute points
– Trivial to spoof mail
– Message path leaks information
– There’s a protocol for asking if an email
address is valid---useful for/against spammers
– Even with S/MIME, header fields not protected
98
Example
• Kerberos Network Auth Service
• Some excerpts
– solves authentication
– does not address authorization or DOS or PFS
– requires on-line database of keys, so NAS must be
physically secured
– subject to dictionary attack (pick good pwds)
– requires reasonably synchronized clocks
– tickets might contain private information
– NAS must remember used authenticators to avoid
replay
99
Agenda
• • • • Introduction to Security
Introduction to Cryptography
Authenticating People
Security mechanisms to reference rather than
invent
– Public Key / Secret Key infrastructures
– Formats
• Security Considerations Considerations
• Security Working Groups
100
Working groups
• btns (better than nothing security)
– Unauthenticated IPsec connections
– Channel bindings
• dkim (Domain Keys Identified Mail)
– Domain signs outgoing email
– DNS indicates domain key, and dkim-enabled
• emu (EAP method update)
• hokey (handover keying)
– tell new access point the session key when client moves
101
IETF working groups
• ipsecme (IPSec maintenance & extensions)
• isms (integrated security model for SNMP)
• keyprov (Provisioning of symmetric keys)
• kitten (GSS-API Next Generation)
• krb-wg (kerberos)
• ltans (long-term archive and notary service)
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IETF working groups
• msec (multicast security)
• nea (network endpoint assessment)
• pkix (public key infrastructure, based on X.
509)
• sasl (simple authentication and security
layer)
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IETF working groups
• smime (S/MIME mail security)
• syslog (security issues in network event
logging)
• tls (transport layer security)
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Conclusions
• Until a few years ago, you could connect to the
Internet and be in contact with hundreds of
millions of other nodes, without giving even a
thought to security. The Internet in the ’90’s was
like sex in the ’60’s. It was great while it lasted,
but it was inherently unhealthy and was destined
to end badly. I’m just really glad I didn’t miss out
again this time. —Charlie Kaufman
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